fix: handle invalid bookmark storage

[saurabhhhcodes]

Summary

• guard bookmark initialization against malformed openCSE_bookmarks localStorage data
• clear corrupted bookmark storage instead of letting JSON.parse crash the page
• ignore malformed bookmark entries while preserving valid saved bookmarks

Fixes #236

Root Cause

The useBookmarks hook directly parsed localStorage.getItem("openCSE_bookmarks") with JSON.parse() during initialization. If that stored value was not valid JSON, React hit a runtime exception before the bookmarks UI could render.

Changes Made

• Added a shared BOOKMARKS_STORAGE_KEY constant
• Added a small parser helper that validates the saved value is an array
• Wrapped stored bookmark parsing in try/catch
• Removed corrupted storage and reset bookmarks to an empty list on parse failure

Testing

• npm run lint passes with existing repo warnings only
• npm run build passes
• git diff --check
• Started local dev server on http://127.0.0.1:3026
• Verified in headless Chrome via DevTools Protocol:
  • set localStorage.openCSE_bookmarks = "abc"
  • reloaded /bookmarks
  • confirmed the page still renders My Bookmarks
  • confirmed the corrupted storage key is cleared
  • confirmed zero runtime exceptions

Impact

Users with corrupted or manually edited bookmark storage no longer get blocked by a bookmarks initialization crash. The app recovers automatically and continues loading normally.

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